There have been recent news accounts of people being overweight (read fat) but still being fit. By fit, let's assume it means the overweight subjects had healthy cardiovascular systems and adequate muscle -- in spite of their excess. And that they were no more likely to contract heart disease, diabetes, etc., than leaner people.
You have to wonder: Just how fat were the test subjects? And exactly how fit were they? Because at some point in being overweight, it is impossible to be what any reasonable person would define as fit or healthy.
The whole "Fat but Fit" concept sounds a little goofy to me. And it may be that researchers have already found signs of it being invalid. Check it out here.